Optical fibers have been widely used as a transmission medium in the field of long-range and medium-range communication for FTTH (Fiber to the Home) or automotive applications. High-speed transmission using light has recently also become a necessity for short ranges of less than 1 m. Optical wiring boards of an optical waveguide type that enable high-density wiring (narrow pitch, branching, crossing, multilayer configuration, etc.), surface mounting, integration with an electric substrate, and small-radius curving, which are the features unattainable with optical fibers, have been used in such a range.
In general, optical wiring boards of the following two types are needed. The optical wiring boards of the first type should be interchangeable with printed wiring boards (PWB), and those of the second type should be interchangeable with flexible printed substrates (FPC) used at hinges of small terminal devices.
Since the optical wiring boards of each type should enable electric wiring and low-speed signal transmission for actuating a VCSEL (Vertical Cavity Surface Emitting Laser) or PH (PhotoDiode), which is a light-receiving element, the ideal board is a photoelectric composite wiring board including an optical circuit and an electrical circuit.
To realize such a configuration, an optical waveguide should be formed on the conventional electrical circuit substrate with a small linear expansion coefficient. Meanwhile, since a resin material constituting the optical waveguide is required to be transparent, no filler can be compounded therewith. For this reason, a resin material with a high linear expansion coefficient is typically used for the optical waveguide. Therefore, an optical waveguide material with a high linear expansion coefficient is laminated on a substrate with a low linear expansion coefficient. The resultant problem is that stresses caused by the difference in linear expansion coefficients appear in the thermal history of the process or thermal history of reliability, and the substrate is warped. As a result of such warpage, stresses are applied to the mounted elements, conduction is disrupted and the chip can be fractured. Accordingly, it is desirable that a transparent resin material with a low linear expansion coefficient be used.
A resin composition in which nanosize particles are compounded with a liquid material is known as a transparent resin composition for an optical material (for example, Patent Documents 1 and 2).
Patent Document 1: Japanese Patent Application Publication No. 2009-235325
Patent Document 2: Japanese Patent Application Publication No. 2009-40850
Meanwhile, a method for forming a liquid material by spin coating or bar coating and a method for laminating a dry film material that is solid at a normal temperature with a pressing device of a vacuum laminator are typically used for forming a core layer or a clad layer constituting an optical waveguide, but the dry film material that can be used for forming a film by a vacuum laminator method is more preferred because of excellent productivity thereof.
Resin compositions including nanosize particles, such as those of the prior art described hereinabove, have been reported as liquid materials, but dry film-shaped materials that excel in transparency are presently unavailable. This is apparently because of the following problem. Thus, where two or more hydroxyl groups are present in one compound from among the resin composition, curing agent, and other additives constituting the dry film, those hydroxyl groups act together with the hydroxyl groups on the surface of nanosize particles, causing cohesion which results in white turbidity.
Where an attempt is made to remove completely the hydroxyl groups by surface treatment of the nanosize particles in order to resolve the aforementioned problem, the hydroxyl groups are difficult to remove completely because of a steric hindrance of the coupling material modifying the surface. Therefore, the white turbidity caused by cohesion is difficult to prevent.